fallacious

Something fallacious is a mistake that comes from too little information or unsound sources. Predictions that the whole state of California will snap off from the rest of North America and float away have proven to be fallacious — for now, anyway.

Fallacious comes ultimately from the Latin fallax, "deceptive." The word fallacious might describe an intentional deception or a false conclusion coming from bad science or incomplete understanding. A tween's assumption that anyone over 20 can't understand her situation would be fallacious; we have all been young once too.